Not sure if this is still active, but I had the same problem on my '87 325is since I bought it 2 years ago. I did all the tests and it came down to the 3rd brake light. Remove the module so you can inspect the printed circuit board (PCB) that has a copper coil soldered on it, and inside the coil, a wire goes through it. On mine, there was a "cold solder" on one of the connections of the inner wire (you would see the lead solder cracked). I just redid the solder to ensure good connectivity, and the check light behaved correctly since then! Hope this helps someone else.